The use of amorphous silicon (a-Si) for optical device applications is rapidly expanding as the telecommunications industry increasingly turns to waveguide-based networks to meet increased demand in bandwidth and speed. And in particular, various optical waveguide architectures and fabrication technologies have been developed employing a-Si in the fabrication process to produce optoelectronic devices at a low cost.
For example, in U.S. Pat. No. 5,841,931 to Foresi et al, a method of forming a polycrystalline semiconductor waveguide for optoelectronic integrated circuits is shown having a polycrystalline core layer 14 surrounded by first and second cladding layers 12, 16 (e.g. SiO2). The polycrystalline semiconductor waveguide of Foresi is produced, for example, by depositing a layer of amorphous semiconductor such as a-Si on the first cladding layer, annealing the a-Si into a polycrystalline semiconductor core layer, surface polishing the polycrystalline semiconductor core layer, and deposition forming the second cladding layer on the polished polycrystalline core. The polycrystalline core layer may be patterned as a strip (FIG. 2 of Foresi) by etching the core layer prior to forming the second cladding layer thereon. A similar waveguide architecture and fabrication method is disclosed in U.S. Pat. No. 5,354,709 to Lorenzo et al. having a polycrystalline core annealed from a-Si and etched to form ribs. In Lorenzo, the polycrystalline core is also bounded by SiO2 cladding layers which are deposition formed separately from the core.
As evidenced in the '931 and '709 patents, amorphous silicon is often utilized for the limited purpose of providing a transitional deposition layer to provide fast, economical deposition onto a substrate at low temperatures. Ultimately, however, the a-Si layer is converted into a polycrystalline core layer (e.g.; by annealing) to produce a polycrystalline silicon (p-Si) waveguide having low refractive index layers (e.g. SiO2) cladding the high index p-Si core layer and deposited separately from the core layer. A fundamental problem with p-Si waveguides, however, is that they characteristically exhibit relatively large losses due to a variety of effects, including grain boundary induced optical scatter.
In contrast, waveguides employing amorphous material in the core are known to exhibit reduced scatter and lower loss than polycrystalline silicon (p-Si) waveguides. One example of such a waveguide and fabrication approach is shown in the referenced publication, “Hot-wire deposition of photonic-grade amorphous silicon” by C. M. Fortmann et al. employing hot-wire deposition to form a thin film of amorphous silicon hydride (a-Si:H). The approach relies on the fact that the refractive index of a-Si:H depends on the H concentration, with the refractive index decreasing with increasing hydrogen concentration. Following a-Si deposition, a mask is used to protect selected areas (the core) and the H concentration of the surrounding regions is raised using ion implantation to form the cladding. Apart from the cost and complexity of the process, this approach is problematic in that H diffusion in a-Si is strongly temperature dependent, with modest temperature increases ˜100 Celsius producing sufficient interdiffusion to eliminate the H concentration gradients over modest periods of time. Consequently the temperature of these structures must be carefully controlled.
Additionally, amorphous silicon has been extensively investigated as an electronic material, and is the focus of continuous technological developments by the VLSI industry due to its compatibility with CMOS processing. For example, laser re-crystallization is currently used to fabricate polycrystalline thin film transistors (TFT) from a-Si films for active matrix displays. For such applications, it is desirable to convert an a-Si film on a glass substrate into p-Si. This is accomplished by scanning a short wavelength laser across the a-Si film. The duration of the illumination is sufficient that the illuminated portion of the a-Si film is raised to the melting temperature; upon cooling the film adopts a poly-crystalline morphology. Typically, to avoid undo heating of the substrate and surrounding regions, a pulsed laser is utilized (usually an excimer laser operating at 308 nm). The laser fluence and pulse duration are adjusted to perform the recrystallization and minimize local heating.
In summary, it would be advantageous to provide an a-Si waveguide architecture and fabrication technology for opto-electronics applications, whereby waveguides fabricated from pure a-Si could operate over significantly larger temperature ranges and could provide robust, low-cost, high bandwidth, high speed, waveguide-based, network and photonic interconnects for the telecommunication industry.